1. Field of the Invention
The present invention relates to insulated gate bipolar transistors.
2. Description of the Background Art
Recently, inverter devices are used in the fields of household electrical appliances and industrial power apparatuses. An inverter device using commercial power supply (AC power supply) includes a converter part for converting AC power supply into DC, a smoothing circuit part, and an inverter part for inverting the DC voltage into AC. As to the main power elements in the inverter part, insulated gate bipolar transistors (hereinafter also referred to as IGBT (Insulated Gate Bipolar Transistor)) capable of high-speed switching are the mainstream.
In an inverter device for controlling power, the current rating and voltage rating per one transistor chip are around in the ranges of several A to several hundreds A and several hundreds V to several thousands V, respectively. Accordingly, in a circuit in which the gate voltage of the IGBT is continuously varied by using a resistance load, the power as the product of current and voltage is generated as heat inside the IGBT, so a large radiator is necessary, and the conversion efficiency of power is poor. Also, depending on the combination of operating voltage and operating current, the temperature of the transistor itself rises and it undergoes thermal breakage, and so a resistance load circuit is not often used.
The load of an inverter device is mostly an electric induction machine (a motor of inductive load), and IGBTs operate as switches and control the power energy by repeating off state and on state. When an inverter circuit is switched with an inductive load, a turn-off process from on state to off state of the transistor, a turn-on process from off state to on state, and on state of the transistor can be thought.
The inductive load is connected to a middle potential point between the upper and lower arms, and the directions of current passed to the inductive load are both positive and negative directions. Current passing through the load is returned from the load connection end to the high-potential power supply side, or passed to the ground side, so free-wheel diodes are necessary to circulate the large current flowing in the inductive load between the load and arm's closed circuit side.
In small-capacity inverter devices, MOSFETs (Metal Oxide Silicon Field Effect Transistors) may be used in place of IGBTs.
As structures for making the on-state voltage of IGBTs small, trench gate IGBTs (see Japanese Patent Application Laid-Open No. 2004-158868 (Patent Document 1)), carrier accumulating trench gate IGBTs, etc. are suggested.
The trench gate IGBT described in Patent Document 1 requires an N-base layer having a certain thickness in order to hold the withstand voltage, but making the N-base layer thick causes the on-state voltage to be high.